This invention relates to signals, and more particularly to measuring delays of circuits that transmit signals.
Circuit delay measurements are widely used to troubleshoot and characterize transmission systems. An example of such a circuit is telephone network 110 in FIG. 1. Network 110 transmits voice and data signals between transmitters/receivers 112 and 114. The signals from transmitter/receiver 112 are transmitted through channel 120 ("transmit channel"). The signals from transmitter/receiver 2 are transmitted to transmitter/receiver 112 through channel 122 ("receive channel"). The delay of a channel or a network is the time that it takes the signal to propagate through the channel or the network. Delays in a typical network vary from near zero to about 1.2 seconds depending on the network complexity and the distances involved.
The delay is measured and compared to its normal value during troubleshooting. The delay measurement helps characterize the system. For example, the delay measurement helps determine the need for an echo canceler. Namely, part of the signal transmitted on transmit channel 120 is reflected back to transmitter/receiver 112. If transmitter/receiver 112 is a telephone set, the person speaking on the telephone hears the reflected signal as an echo. If the echo is delayed by 10 milliseconds or more, the person hears the echo as distinct from his own voice. The echo can be annoying. Echo cancelers are circuits that eliminate echoes. The circuit designer determines from delays of the network or its components whether an echo canceler is needed.
FIG. 2 illustrates how the "round trip" delay through a network is measured. Signal generator 210 sends a signal through transmit channel 120. The signal typically has a carrier frequency of 300 Hz to 3004 Hz, which is an appropriate range for voice and data signals. The carrier is modulated at f.sub.m =83.33 Hz. At the other end 220, the signal is looped back and returned on receive channel 122. The time from sending the signal 410 and receiving it back on channel 122 is measured, providing the round trip delay through network 110.
Direct time measurements are typically imprecise and thus often inadequate. More precise delay values are obtained by comparing the phases of the respective modulation components signals. FIG. 3 illustrates this technique. FIG. 3(a) is a diagram of the modulation component of the signal from generator 210. FIG. 4(b) is a diagram of the modulation component of the returned signal. The time is measured along the horizontal axis, and the amplitude is measured along the vertical axis. Both components have a frequency of 83.33 Hz. The phase offset is shown as .theta..sub.o. The delay d is then computed from the formula EQU d=.theta..sub.o /.omega..sub.m
where .omega..sub.m =f.sub.m .times.2.pi. is the radian frequency of the components.
Since the phase offset .theta..sub.o can be measured only up to 360.degree. (2.pi. radians), the maximum delay d that can be measured is the period of the f.sub.m signal. At f.sub.m =83.33 Hz, the period is 12 milliseconds. The 12 millisecond maximum, however, is unacceptable for many networks. Delays in networks with satellite links often reach and exceed 1,200 milliseconds. In order to measure delays up to 1,200 milliseconds, frequency f.sub.m must be reduced 100 times to below 1 Hz. However, the measurements at such low frequencies take a long time to perform.
Thus there is a need for a technique that would allow to measure delays of over 1 second both precisely and fast.